1. Field of the Invention
This invention relates to a synthetic vascular prosthesis having an antithrombogenic hydrogel layer on the inner surface of the prosthesis and also to a method for manufacturing such a prosthesis.
2. Description of the Prior Art
Synthetic vascular prostheses that are currently in practical use are all of the neointima formation type, so that after implantation into a living body, thrombi are immediately formed on the inner surface of the prosthesis. When the thrombi are developed to cover the inner walls of the synthetic vascular prostheses in a certain thickness, further thrombus is not formed, thus ensuring the flow of blood. Subsequently, the neointima grow whereby the synthetic vascular prosthesis acquires the antithrombogenic property, thus performing the function of a blood vessel. The synthetic vascular prosthesis of this type, however, has a disadvantage that it is not applicable to a small prosthesis having an inner diameter of about less than 4mm. This is because the prosthesis is plugged by the thrombi prior to the formation of the neointimae, thus preventing an effective patency of the prosthesis.
On the other hand, research and development have recently been made on a synthetic vascular prosthesis of the type which has an antithrombogenic property by itself and thus does not need the formation of the neointima. These synthetic vascular prostheses are generally divided into two kinds. In one kind, a living body-derived anticoagulant such as heparin, urokinase or the like is impregnated in, or is fixed through covalent bonds or ionic bonds to, a synthetic vascular prosthesis substrate so as to impart the antithrombogenic property thereto. In the other kind, a hydrogel layer is formed on the inner surface of a synthetic vascular prosthesis so as to prevent direct contact between a substrate of the prosthesis and proteinous and cellular components in the blood, thereby acquiring the antithrombogenic property. The prosthesis of this kind is disclosed in, for example, Japanese Patent Publication No. 60-242857.
In the former instance, however, the impregnated anticoagulant will flow away and be lost, or if fixed, the anticoagulant will gradually lose its activity and efficacy. Therefore, an everlasting antithrombogenic property cannot be obtained.
The latter kind where the hydrogel layer is formed on the inner surface of the synthetic vascular prosthesis, is believed to be more suitable since the antithrombogenic property can be maintained substantially permanently. Further, the synthetic vascular prosthesis of this kind is applicable to a small-diametered portion of the blood vessel as the neointimae need not be formed.
It is known that a hydrogel layer has a good antithrombogenic property and is formed by graft polymerization on the inner surface of a tubular base member which is made of elastomer, typically polyurethane. The base member should be porous in order to render the prosthesis flexible and to facilitate a joining operation of the synthetic vascular prosthesis to a natural vessel by suture. The porous base member, however, permits the hydrogel to impregnate therethrough, which is not desirable because it promotes calcification throughout the base member and reduces the flexibility of the prosthesis. It has thus been proposed to provide a thin, non-porous, dense layer between the inner surface of the base member and the hydrogel layer, so that the hydrogel may be prevented from infiltrating into the base member. However, fabrication of the hydrogel layer on such a dense layer requires complicated operations such as plasma treatment of the inner surface of the dense layer under vacuum to generate radicals, graft polymerization of a hydrophilic monomer (acrylamide or the like) on the inner surface, and removal of the resultant homopolymer not grafted. In addition, expensive apparatuses such as a vacuum pump, a high frequency generator and the like are necessary for the plasma treatment.
Furthermore, the provision of a dense layer increases the stiffness of the prosthesis and reduces the compliance. The compliance, of which measurement will be described later, is a value which indicates a variation in inner capacity of the prosthesis when an internal pressure is exerted thereon. A larger value results in a larger variation in the inner capacity when the internal pressure is constant. Generally, the compliance of the synthetic vascular prosthesis is small compared with that of a natural blood vessel. The difference in compliance between the synthetic vascular prosthesis and the natural vessel, tends to develop an aneurysm at the joint or inosculated portion or to break the prosthesis thereat.
Accordingly, an object of the present invention is to provide a synthetic vascular prosthesis which has a satisfactory strength and compliance as well as a good antithrombogenic property.
Another object of the invention is to provide a method for manufacturing a synthetic vascular prosthesis of the type set forth above by relatively simple operation.